There have conventionally been variously investigated a W/O/W type emulsion for use as a medicine in which a drug is encapsulated within an internal aqueous phase; a cosmetic preparation which can provide smooth feeling when it is applied onto the skin; and a food having a low calorie. However, the W/O/W type emulsion suffers from a problem in that it is insufficient in its stability as compared with O/W type one or the like. One of the reasons for this is that salts and saccharides present in an internal aqueous phase and an outer aqueous phase may serve to establish an osmotic pressure difference between them and this in turn results in the transfer or exchange of water between them through an oil phase. In particular, when the emulsion of this type is applied to a food product, a variety of components interact with one another in a complicated manner and accordingly, it has been believed that there have not yet been proposed any dressing in the form of a W/O/W type emulsion having excellent storage stability which allows the dressing to withstand currently used various conditions encountered when articles on the market are distributed and stored, to say nothing of such an emulsion stored or managed in a laboratory in which conditions such as temperature and raw materials would be strictly controlled.
There have been known methods for preparing a W/O/W type emulsion for use in a food such as dressing and mayonnaise, whose acidity or sourness is reduced (see Patent Document 1 given below). According to this invention, the emulsion is characterized in that the amount of water present in an internal aqueous phase thereof is greater than that present in an outer aqueous phase and that the amount of an acidulant contained in the internal aqueous phase is greater than that contained in the outer aqueous phase. For this reason, the emulsion is greatly limited in its applications when using the same as a food in view of taste and palatability as well as production cost thereof. Moreover, Patent Document 1 simply discloses currently known information about polyglycerol polyricinoleate.
There has also been known a method for the preparation of a W/O/W type composite emulsion which comprises, as an essential element, adding a polyglycerol polyricinoleate to an internal aqueous phase of the emulsion (see Patent Document 2 given below). This invention aims at the preparation of dressing and mayonnaise, which are excellent in their stability. However, any polyglycerol polyricinoleate can be used in this invention and there is not disclosed any limitation on the degree of esterification and the degree of condensation of ricinoleic acid and there is not likewise any limitation in the amount thereof to be incorporated. It was found to be difficult for the invention disclosed in this document, to prepare any W/O/W type composite emulsion excellent in its storage stability to such an extent that it can be distributed as an article on the market.
There has also been known an invention concerning dressing in the form of a W/O/W type composite emulsion (see Patent Document 3 given below). Patent Document 3 discloses a polyglycerol polyricinoleate as an example of an emulsifying agent used therein. However, this patent document simply discloses general knowledge about W/O/W type composite emulsion.
There has also been known a method for the preparation of a W/O/W type composite emulsion characterized in that a lipophilic polyglycerin fatty acid ester is incorporated into the emulsion (see Patent Document 4 given below). Patent Document 4 discloses a hexaglycerol polyricinoleate as an emulsifying agent usable in the invention. However, this patent document states that any emulsifying agent other than the polyglycerol polyricinoleate may be used in the W/O/W type composite emulsion and accordingly, it is not believed that the resulting product or emulsion will have satisfactory stability.
There has likewise been known a method for preparing a W/O/W type composite emulsion which is characterized in that water, an oily component and a lipophilic emulsifying agent are incorporated into the emulsion in a specific mixing ratio and which can maintain a high rate of a drug to be encapsulated into an inner aqueous phase of the composite emulsion (see Patent Document 5 given below). However, it is essential in this invention to use the lipophilic emulsifying agent in an amount of not less than 20% and not more than 66% and accordingly, it is recognized that it is quite difficult to use the emulsion as an ingredient for a food while taking into consideration production cost, taste and palatability thereof as well as method for the preparation of the same.
There has been known a mushroom extract-containing composite emulsion characterized in that it comprises 0.5 to 10% by mass of a polyglycerol polyricinoleate whose glycerin-condensation degree corresponds to dimer to hexamer, relative to oil and fat component (see Patent Document 6 given below). It is an object of this invention to mask bitterness or a bitter taste of the extract derived from mushroom and the invention is characterized in that an aqueous solution containing the mushroom extract is gradually added to and dispersed in an oil component to thus form a W1/O type emulsion, and the resulting emulsion is then dispersed in water (W2) to thus give a W1/O/W2 type emulsion. This patent document specifies the degree of ricinoleic acid-condensation in the polyglycerol polyricinoleate used therein and the degree of glycerin-condensation of the polyglycerin, but the document does not disclose or teach the degree of esterification thereof.    Patent Document 1: WO2004/047562    Patent Document 2: JP-A-60-203139    Patent Document 3: JP-A-60-184366    Patent Document 4: JP-A-59-173131    Patent Document 5: WO 2002/043698    Patent Document 6: JP-A-2005-185180